In the field of medical engineering, many devices are known with which fluids can be withdrawn from patients or delivered to the patients via a tubular conduit. The access to the patients is usually made with catheters for insertion into organs of the body, or with cannulas for puncturing vessels. During the examination or treatment, correct access to the patient has to be ensured. It is therefore necessary to monitor the patient access. In blood purification methods such as hemodialysis, hemofiltration and hemodiafiltration, a patient's blood is passed through an extracorporeal blood circuit which comprises a membrane filter, for example a dialyzer, or a hemofilter or hemodiafilter divided by a semi-permeable membrane into a blood chamber and a separate dialysis fluid chamber or filtrate chamber via which the filtrate drains off. In a hemodiafiltration machine in which both hemodialysis and hemofiltration are performed simultaneously, the draining of filtrate also takes place at the same time on the dialysate side. If the venous connection to the patient comes loose during the blood treatment, bleeding to death can be avoided only if the extracorporeal blood flow is stopped within a few seconds. Therefore, the extracorporeal blood circuit of the known blood treatment devices is generally provided with protective systems which, in the event of an alarm, stop the blood pump, close the venous tube clamp and emit an acoustic and/or optical warning signal.
DE 197 39 099 C1 describes a device for monitoring a vascular access during an extracorporeal blood treatment, in which an electric current is induced in the connection of the extracorporeal blood circuit representing a closed conductor loop, the current flowing in the conductor loop is measured, and a characteristic change in the current strength points to a faulty vascular access. In addition to inductive injection and output, it is also known, from document U.S. Pat. No. 6,932,786 B2, to perform capacitive injection and output of electric signals in the extracorporeal blood circuit.
WO 99/29356 describes a device for monitoring a vascular access, with a puncture cannula to which a tubular conduit is connected that leads to a fluid source. For monitoring the vascular access, an electrical signal is injected into the fluid flow downstream of the fluid source, at a first location of the tubular conduit, and an electrical signal is output from the fluid flow at a second location downstream of the first location. The electrical signal, however, can also be output at a location on the patient's skin. If the output electrical signals are greater than a predetermined limit value, an alarm is triggered.
US 2003/0195454 A1 deals with the problem of capacitive injection and output of measurement signals in the extracorporeal blood circuit and proposes injection and output of the measurement signals by means of electrical contact elements that are in contact with the blood flowing through the tubular conduits.
US 2006/0081517 A1 discloses a device for monitoring the arterial and venous vascular access during an extracorporeal blood treatment with a hemodialysis device in which a predetermined voltage is applied to the dialysis fluid circuit and blood circuit. The voltage lies between two electrodes, of which one is in contact with the blood flowing in the blood circuit, and of which the other is in contact with the dialysis fluid flowing in the dialysis fluid circuit, so that there is an electric current flowing in the blood circuit and in the dialysis fluid circuit. The known device assumes that the peristaltic blood pump arranged in the blood circuit constitutes an interruption of the electric circuit.
The devices and methods described above have the disadvantage that electrical signals have to be injected into the fluids, as a result of which the outlay required for monitoring the patient access is increased. In addition, the injection of the electrical signals into the flow of fluid has proven problematic in many ways.
“Method for detecting the disconnection of an extracorporeal device using a patient's endogenous electrical voltages,” by Ross et al. in Kidney International (2006) 69: 2274-2277 describes a method for monitoring a patient access in which electrical measurement signals are not injected into the flow of fluid, and instead it is only endogenous electrical voltages that are evaluated. The method by Ross et al. requires an electrical connection to be produced between two electrodes and the blood. The endogenous electrical signals that are picked up on the venous and arterial tubular conduits are almost identical because the measurement electrodes are at a distance measuring only a few centimeters. If the vascular access to the patient is as it should be, the difference of the measured signals is approximately zero. By contrast, in the case of a faulty vascular access, a potential difference can be demonstrated.
The known method is therefore based on monitoring the potential difference of endogenous signals, and the injection of electrical measurement signals into the flow of fluid is not necessary. However, a detection of ECG (electrocardiogram) signals with a voltage measurement at closely adjacent measurement points would be scarcely possible with the known method, because the so-called “heart vector” cannot be adequately recorded with such close spacing of the electrodes. For this reason, only diffuse endogenous signals could be detected by the measurement electrodes. During the measurement, interference signals from the environment are also inevitably measured, for example ripple voltages (50 Hz or 60 Hz) that are superposed on the supply voltage.
WO 2004/108206 A1 deals with the problem whereby occlusive blood pumps generate interference signals that are superposed on endogenous signals. This problem arises particularly when measuring ECG signals while the blood pump of an extracorporeal blood treatment device is running It is also known from WO 2004/108206 A1 that the dialyzer of a hemodialysis device does not constitute a barrier to an electric current flow between blood circuit and dialysis fluid circuit.
“Study on causes and prevention of electrostatic charge build-up during extracorporeal circulation,” by Snijders et al. in Perfusion 1999; 14: 363-370 deals with the causes of the electrostatic charging in extracorporeal blood circuits and with their prevention, while “Investigation of the phenomenon of electrostatic compromise of a plastic fiber heat exchanger,” R. J. Elgas, in Perfusion 1999; 14: 133-140, deals with the phenomenon of electrostatic charging in a heat exchanger with synthetic fibers.